The present invention relates generally to the writing of images and, more particularly, to the writing of images in liquid crystal cells using a laser beam.
It is well known to form visual images by exposing a photosensitive media to light carrying the image information to be recorded. Exposure light is directed to the media after it has been reflected from or transmitted through an information source. Such a source may be a photomask, with one specific example being a mask formed by a laser addressable liquid crystal (LALC) cell.
It is also known to write image information onto liquid crystal cells using a scanned laser beam modulated in accordance with the image to be written. When forming images in a photosensitive media, the written liquid crystal cell is used as a mask for transmission or reflection of exposure light. Examples of such systems may be found in Dewey, "Laser-Addressed Liquid Crystal Displays," Optical Engineering 23(3), 230-240 (May/June 1984).
If a color image is to be reproduced in the media, at least three liquid crystal cells are used, each cell forming a mask corresponding to one of the three primary colors. The three masks may then be used in exposing the media, either simultaneously or sequentially, to exposure light of the primary colors.
For color images, it is typical to use a single laser writing system to write the three laser addressed liquid crystal (LALC) cells. The cells are written sequentially so that the overall writing time for a complete image is relatively lengthy, i.e., three times that required for a single cell. Simply providing three writing systems is impractical, particularly due to the expense of the galvo mirror typically used in laser scanning and the lens system used to focus the beam onto the mask surface.
A further problem in writing multiple images corresponding to color components of a full-color image is that proper registration between the multiple cells must be achieved. This is true regardless of whether a single writing system is used sequentially or multiple independent systems are used simultaneously. Otherwise, the finished color image may exhibit misregistration between the colors. This has proved difficult, however, due at least in part to drifting of the galvanometer mirror typically used in laser scanning, as well as non-repeatability of cell positioning.
What is needed, therefore, is a method and apparatus for writing multiple images simultaneously onto multiple cells or discrete areas of a single cell. Registration between the images should be precise and repeatable. Further, the method and apparatus should be of reasonable cost and complexity.